Suction device for separating and/or transporting sheets in a sheet-handling machine

ABSTRACT

The suction device includes a pneumatic cylinder and a hollow piston in the cylinder movable between a retracted and a nonretracted position and comprised of axially adjacent first and second portions, at least a part of the circumferential periphery of each of the two portions being spaced from and out of contact with the inner periphery of the cylinder to define therewith respective first and second spaces. The piston unit includes a sheet-engaging portion having a suction opening which communicates with the ambient atmosphere and constitutes the end of the hollow interior of the piston unit, the hollow interior communicating with the second space via the first space. The cylinder is provided with a suction port opening into the second space to form a path for the flow of ambient air into the suction opening and out through the suction port, the flow cross section of the first space being smaller than that of the second space so that the first space constitutes a flow restriction in such path.

[451 Nov. 25, 1975 SUCTION DEVlCE FOR SEPARATING Primary ExaminerEvon C. Blunk Assistant Examiner-Robert Saifer AND/OR TRANSPORTING SHEETS IN A SHEET-HANDLING MACHINE Attorney, Agent, or Firm-Michael S. Striker inventor:

Karl-Hans Vollrath, Leipzig, Germany [57] ABSTRACT The suction device includes a pneumatic cylinder and [73] Assignee: VEB Polygraph Leipzig Kombinat fur PolygraphischeMaschinen und a hollow piston in the cylinder movable between a re- Ausrustungen, Leipzig, Germany tracted and a non-retracted position and comprised of June 28, 1974 Appl. No.: 484,452

[22] Filed axially adjacent first and second portions, at least a part of the circumferential periphery of each of the two portions being spaced from and out of contact' with the inner periphery of the cylinder to define therewith respective first and second spaces. The pis- [30] Foreign Application Priority Data Aug. 4, 1973 ton unit includes a sheet-engaging portion having a suction opening which communicates with the ambient atmosphere and constitutes the end of the hollow interior of the piston unit, the hollow interior communicating with the second space via the first space. The

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I108 cylinder is provided with a suction port opening into the second space to form a path for the flow of ambi- [56] References Cited FOREIGN PATENTS OR APPLICATIONS ent air into the suction opening and out through the suction port, the flow cross section of the first space being smaller than that of the second space so that the 716,791 United Kingdom................. 271/103 first space constitutes a flow restriction in such path.

10 Claims, 3 Drawing Figures A II II NEE/477N611 EAA/S' lll/IIIIII I: IIfllIlI/I/Il/lll/ll/l I ll US. Patent Nov. 25, 1975 3,921,971

SUCTION DEVICE FOR SEPARATING AND/OR TRANSPORTING SHEETS IN A SHEET-HANDLING MACHINE BACKGROUND OF THE INVENTION The invention relates to a sheet-separating and/or sheet-transporting suction device of the type conventionally used in sheet-handling machinery, such as in printing machines, copying machines, and the like.

A variety of such sheet-separating and/or sheet-transporting suction devices of this type is already known, especially for removing sheets from a stack of sheets, and for then feeding the successively removed sheets onto a conveyor which transports the sheets into a printing machine, or the like. These sheet-separating and/or sheet-transporting suction devices are synchronized with the operations of the associated sheet-handling machine and are pneumatically controlled by the synchronized application and removal of operating suction. This timed application and removal of operating suction is for example customarily synchronized with the movement of a holding arrangement on which one or more of such suction devices are mounted; for example, the suction devices engage the top sheet of a stack and the holding arrangement moves along a predetermined path to a location above a conveyor belt, whereupon the operating suction is removed and the sheet falls onto the conveyor belt, the holding arrangement then performing a return movement back to the stack for the engagement of the next sheet.

It is disclosed in a variety of publications (amongthem West German published Pat. application 2,132,434) to provide the suction devices in a fixed housing, and in particular in such a manner that the suction devices are mounted on pistons which can be raised and lowered, the pistons being returned to their upper starting positions by biasing springs when the operating suction is removed.

Another such sheet-separating and -feeding suction device for pneumatic sheet-handling machinery is also disclosed in West German petty Pat. 7,022,786. In this construction, likewise, a compression spring holds a piston in the upper position thereof, the piston carrying the sheet-en gaging suction nozzle; between the suction chamber for the movement of the piston into the operating position, i.e., for the downward movement of the piston into Contact with the stack of sheets, and the suction chamber for the upwards movement of the piston upon closing off of the suction nozzle of the suction device by the actual sheet which is to be removed from the stack and/or transported or fed, there is formed a flow restriction passageso configurated that in the vicinity of the piston a bore is arranged in the piston rod. In order to avoid a jamming of the piston with thislatter construction, as well as with all the known constructions, the surface portions of the piston and those of the inner wall of the cylinder which contact each other during relative sliding movement therebetween must be of high quality material and manufactured in a very precise manner, with narrow tolerances. Each of the known designs includes at least two working or slidingcontact surfaces, placing high demands upon the precision of the shape and mounting of the relatively movable parts; the necessarily narrow tolerances require a relatively complicated method of construction which is not advantageous from the manufacturers point of view.

, ference' principle of the prior-art devices, but which SUMMARY OF THE INVENTION One object of the invention is to provide a sheetseparating and/or sheet-feeding or -transporting suction device which avoids the disadvantages of the priorart devices.

Another object of the invention is to provide such a suction device which does not depart altogether from the basic structure of the prior-art devices, but which is different in respects which permit the manufacture of the suction device in an uncomplicated manner, in which relatively large tolerances are permissible, and which lends itself readily to the formation of the suction device from plastic. In particular, it is considered very important tobe able to design a suction device of the type in question which operates on the pressure-difcan be made of plastic material in a simple manufacturing process, and which after being manufactured and installed requires virtually no maintenance.

These objects, and others which will become more understandable from the following description of a preferred embodiment of the invention, can be set, according to one advantageous concept of the invention, by providing a sheet-separating and/or sheet-transporting or feeding suction device in which the flow-restricting passage hitherto provided in the form of a bore in the piston rod is replaced by a flow-restricting passage in the form of an annular clearance between the piston and the surrounding cylinder, with the piston rod of the piston being guided for sliding movement in a simple elongated piston rod guide member. It is advantageous to furthermore provide adjacent to the piston rod, in the region of the suction inlet of the suction device, and inparticular in the region of the elongated piston rod guide member, at least one suction-release passage extending in direction parallel to the elongated piston rod, the suction release passage terminating just above the upper edge of the piston rod guide member, when the piston is in the upper (retracted) position thereof.

One advantage of this new construction is that the sheet-separating and/or -transporting and/or -feeding suction device is inherently of low mass and suitable for manufacture from plastic. In addition, compared to known suction devices of this general type, the arrangement of the suction release passage mentioned above has the advantage that when the suction-generating device is turned off, the suction within the suction device almost immediately terminates, so that the sheet being held by such suction force almost immediately drops; on the other hand, whenthe suction-generating device is on, the existence of the suction release passage does not detract from the reliability with which the sheet is held by the suction nozzle of the device. As a result, the operating cycle of the sheet-separating and/or sheettransporting or feeding device can be gone through repeatedly with the very high precision and at very high speed.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional object and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference numeral 1 designates the cylinder of the sheet-transporting and/or -separating device. Arranged inside the cylinder 1 is a piston 2 integral with a piston rod 3. The piston rod 3 and the piston 2 are provided with an internal suction conduit 3a. The piston rod 3 is slidable in a relatively long piston rod guide member 4. Arranged intermediate the piston 2 and the piston rod guide member 4 is a compression spring 5 which exerts upon the piston 2 a force tending to urge the latter to the uppermost, retracted position, shown in FIG. 1. Thepiston rod 3 isprovided with a longitudinally extending recess 6 which together with the adjoining surface portions of the piston rod guide member 4 forms a suction release passage. This suction release passage 6 extends from the sheet-engaging suction nozzle 8 up into the cylinderl and terminates shortly past the upper edge 7 of the piston rod guide member 4. Instead of employing a single such suction release passage 6, it would also be possible to provide a plurality of such suction release passages, but with the combined flow cross-sections of all of the passages equalling the flow cross-section of the suction release passage used when only a single such passage is employed. The sheetengaging suction nozzle 8 is arranged at the lower end of the piston rod 3 and is held in place by the radially compressive force of the surrounding elastic members, which for example are made of elastically resilient plastic; this manner of assembly makes for easy removal of the suction nozzle 8 and replacement by another such nozzle, when this is necessary.

The piston rod guide member 4 is secured to the cylinder l by means of one or more mounting screws 9, for

which purpose the cylinder 1 is provided with one or more mounting collars 10. One such collar 10 (such as the one depicted in FIG. 1) can be made considerably longer than otherwise necessary, so as to form a mounting sleeve, and serve for the securing of the entire suction device to a (non-illustrated) supporting framework of a sheet transporting arrangement, or the like. For the sake of fast and reliable assembly of the piston rod guide member 4 with the other parts of the structure, the member 4 has embedded therein one or more radially outwardly projecting pins ll. As shown particularly clearly in FIG. 2, the radially outwardly projecting pins 11 can be introduced into L-shaped slots 12 in the lower portion of the peripheral wall of the cylinder 1 to form with such L-shaped slots 12 bayonet-type connections.

The upper end of the cylinder 1 is provided on its inner surface with a plurality of abutment members 13 which serve to establish an upper end position for the piston 2 against the upwards pushing force of the compression spring 5.

The operating suction connector port 14 is so arranged on the lower part of the cylinder 1 that the piston 2 can be drawn by the force of suction down to the level of the upper edge 7 of the piston rod guide member 4.

Between the outer periphery of the piston 2 and the inner periphery of the cylinder 1 there is defined an annular clearance 15 the flow cross-sectional area of which is substantially smaller than the flow cross-sectional area of the space surrounding the piston rod 3a.

In the event that use is to be made of a suction nozzle 8 having a geometrical form requiring a definite radial guidance (of the piston 2 and accordingly of the suction nozzle 8), then one can provide on the inner wall of the cylinder 1 a radially inwardly recessed depression 17 likewise extending in direction parallel to the elongation of the cylinder 1; such an arrangement prevents the piston 2 from turning when performing its strokes.

The structure depicted in FIGS. 1-3 operates as follows:

A suction-generating device connected to the operating suction connector port 14 is turned on, thereby establishing suction inside the cylinder 1. Air accordingly flows into the suction nozzle 8 through the suction passage 3a, radially outwards over the upper rim of piston 2, axially downwards through the annular clearance 15, axially downwards through the annular space surrounding the piston rod 3, and out through the operating suction' connector port 14. Also, air is initially drawn in through the suction release passage 6 and flows out through the suction connector port 14. Because the flow cross-sectional area of the annular clearance 15 is much smaller than that of the annular space surrounding the piston rod 3, and much smaller than that of the operating suction connector port 14, a greater volume of air flows out through operating suction connector port 14 than can flow axially downwards through the small annular clearance 15. Accordingly, the pressure below the piston rod 2 is markedly lower than the presinto the suction nozzle 8 evidently terminates. As a result, the difference between the pressures above and below the piston 2 decreases towards zero. As a result there is no longer a net suction force difference greater than the biasing force of compression spring 5, and therefore the piston 2 is pushed upwards by the spring 5towards the fully retracted position of FIG. 1. Meanwhile, the sheet engaged by the suction nozzle 8 is still firmly held by the suction nozzle, and accordingly is lifted as the piston 2 moves upwards to its retracted position. When the piston 2 reaches the uppermost position thereof, shown in FIG. 1, the suction release passage 6 becomes unblocked, and ambient air begins to flow through passage 6 into the interior of cylinder 1 and out through port 14; this flow of air, however, is relatively small. When it is desired that the sheet held by suction nozzle 8 be dropped, the suction-generating device connected to port 14 is turned off. Ambient air enters into the cylinder 1 through the suction release passage 6. The subambient pressure inside cylinder 1 very quickly rises to ambient pressure, and the sheet drops down from suction nozzle 8. It will be understood so that when the sheet thusly drops it will drop onto the conveyor belt and, for example, be fed into a copying the present invention.

machine, or the like. The means for holding and moving the illustrated suction device are per se conventional; for example, the suction device could conceivably be mounted as shown in the drawing'on a guide track and driven by a (non-illustrated) drive chain back and forth between the stack and the conveyorbelt. The flow cross-sectional area of the suction release passage 6 is so small, compared to that of port 14, that, even when passage 6 is fully unblocked, the sheet held by nozzle 8 will not be released until the operating suction applied to port 14 is removed. On the other hand, the flow cross-sectional area of the passage 6 is sufficiently great, compared to the volume of cylinder 1 and taking into account the maximum suction developed, that when the suction applied to port 14 is removed the sheet held by nozzle 8 drops virtually immediately. When no suction is applied to the port 14, the compression spring 5 of course maintains the piston 2 in the uppermost, fully retracted position thereof.

it will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in a suction device for the separating and/or transportingand/or feedingof sheets, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of I Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of the prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In a sheet-handling machine, a suction device for engaging and then holding sheets, comprising, in combination, a pneumatic cylinder; a hollow piston units in said cylinder movable between a retracted and a nonretracted position and comprised of axially adjacent first and second portion, at least a part of the circumferential periphery of each of said portions being spaced from and out of contact with the inner periphery of said cylinder to define therewith respective first and second spaces, said piston unit including a sheetengaging portion having a suction opening which communicates with the ambient atmosphere and co stitutes the end of the hollw interior of said piston unit,

said hollow interior communicating with said second space via said first space, said cylinder being provided with a suction port opening into said second space to form a path for the flow of ambient air into said suction opening and out through said port, the flow cross section of said first space being smaller than that of said second space so that said first space constitutes a flow restriction in said path; and drive means for causing said piston unit to move from retracted to nonretracted position and then back to retracted position, in order to cause said sheet-engaging portion to move into engagement with a sheet located in the path of movement of the latter and then hold said sheet by suction during the movement of said piston unit back to said retracted position, said drive'means comprising suction-generating means connected to said port and operativefor establishing subambient pressure in said secondspace, whereby said piston movesfrom said retracted to. said non-retracted position upon the establishment of such subambient pressure as a result of the initial: differenceuin pressures upstream and downstream of said first space andwhereby said piston returns to said retracted position due to the pressure equalization resulting when the sheet engaged and held by said'sheet-engaging portion blocks further entrance ,of ambient air into said suction opening of the said sheet-engaging portion, wherein said first portion of said piston unit is cylindrical and constitutes a'hollow piston portion and wherein said second portion of said piston unit is cylindrical and constitutes a hollow piston rod portion,and-wherein sa'idhollow piston portion is spaced from and out of contact with the inner periphery of'said'c'ylinder all around the circumferentialperiphery of said hollowpiston-portion', whereby there is no frictional wear of the circumferential surface of said piston portion and the facing inner surface of said cylinder, and wherein said cylinder is further provided with an axially extending guide portion for guiding said piston rodportion, said guide portion having an axial end portion facingsaid pistonportion, and whereinsaid said cylinder movablebetween a retracted and a non- ,retracted position and comprised of axially adjacent first and second portions, at least a part of the circumferential periphery of each of said portions being spaced from and out of contact with the inner periphery of said cylinder to define therewith respective first and second spaces, said piston unit including a sheetengaging portion having a suction opening which communicates with the ambient atmosphere and constitutes the end of the hollow interior of saidpiston unit, said hollow interior communicating with said second space via said first space, said cylinder being provided with a suction port opening into said second space to form a path for the flow of ambient air into said suction opening and out through said port, the flow cross section of said first space being smaller than that of said second space so that said first space constitutes a flow restriction in said path; and drive means for causing said piston unit to move from retracted to nonretracted position and then back to retracted position, in order to cause said sheet-engaging portion to move into engagement with a sheet located in the path of movement of the latter and then hold said sheet by suction during the movement of said piston unit back to said retracted position, said drive means comprising suction-generating means connected to said port and operative when activated for establishing subambient pressure in said second space, whereby said piston unit moves from said retracted to said non-retracted position upon the establishment of such subambient pressure as a result of the initial difference in pressures upstream and downstream of said first space, and whereby said piston unit returns to said retracted posi- 7 tion due to the pressure equalization resulting when the sheet engaged and held by said sheet-engaging portion blocks further entrance of ambient air into said suction opening of said sheet-engaging portion. the hollow interior of said piston unit and the part of the hollow interior of said cylinder external to said piston unit together forming when said suction opening is blocked by a sheet and said piston unit is in the retracted position a closed chamber sealed off from the ambient atmosphere to such an extent that the subambient pressure established in such a chamber by said suction-generating means maintains said piston unit in the retracted position indefinitely until said suction-generating means is deactivated.

3. In a sheet-handling machine as defined in claim 2,

wherein said first space is annular and wherein said second space is annular.

4. In a sheethandling machine as defined in claim 2, wherein said first and second portions of said piston I unit are cylindrical and coaxial, with the diameter of said first portion being larger than the diameter of said second portion, and wherein said first and second spaces are both annular. I

5. in a sheet-handling machine as defined in claim 2,

vwherein at leastone of said first and second portions of said piston unit is spaced from and out of contact with the inner periphery of said cylinder all around the cir-,

ery of said cylinder all around the circumferential pe-- riphery of said second portion.

8. in a sheet-handling machine as defined in claim 2, wherein said first space constitutes the only flow restriction in said path.

9. In a sheet-handling machine as defined in claim 2, wherein said cylinder is further provided with a suction release opening having a flow cross-sectional area so 'tering said suction release passage can quickly destroy the subambient pressure in said second space.

10. In a sheet-handling machine as defined in claim 2,

wherein said first portion of said piston unit is cylindrical and constitutes a hollow piston portion and wherein said second portion of said piston unit is cylindrical and constitutes a hollow piston rod portion, and wherein said hollow piston portion is spaced from saidout of contact with the inner periphery of said cylinder all around the circumferential periphery of said hollow piston portion, whereby there is no frictional wear of the circumferential surface of said piston portion and, the facing inner surface of said cylinder, and wherein said cylinder is further provided with an axially extending guide portion for guiding said piston rod portion, said guide portion having an axial end portion facing said piston portion, and wherein said piston rod portion is further provided with a recess constituting a suction release passage having one end opening into the ambient atmosphere and having another end which terminates just past said axial end portions 'of said guide portion when said piston unit is in said retracted position. 

1. In a sheet-handling machine, a suction device for engaging and then holding sheets, comprising, in combination, a pneumatic cylinder; a hollow piston units in said cylinder movable between a retracted and a non-retracted position and comprised of axially adjacent first and second portion, at least a part of the circumferential periphery of each of said portions being spaced from and out of contact with the inner periphery of said cylinder to define therewith respective first and second spaces, said piston unit including a sheet-engaging portion having a suction opening which communicates with the ambient atmosphere and constitutes the end of the hollw interior of said piston unit, said hollow interior communicating with said second space via said first space, said cylinder being provided with a suction port opening into said second space to form a path for the flow of ambient air into said suction opening and out through said port, the flow cross section of said first space being smaller than that of said second space so that said first space constitutes a flow restriction in said path; and drive means for causing said piston unit to move from retracted to non-retracted position and then back to retracted position, in order to cause said sheet-engaging portion to move into engagement with a sheet located in the path of movement of the latter and then hold said sheet by suction during the movement of said piston unit back to said retracted position, said drive means comprising suction-generating means connected to said port and operative for establishing subambient pressure in said second space, whereby said piston moves from said retracted to said non-retracted position upon the establishment of such subambient pressure as a result of the initial difference in pressures upstream and downstream of said first space, and whereby said piston returns to said retracted position due to the pressure equalization resulting when the sheet engaged and held by said sheet-engaging portion blocks further entrance of ambient air into said suction opening of the said sheet-engaging portion, wherein said first portion of said piston unit is cylindrical and constitutes a hollow piston portion and wherein said second portion of said piston unit is cylindrical and constitutes a hollow piston rod portion, and wherein said hollow piston portion is spaced from and out of contact with the inner periphery of said cylinder all around the circumferential periphery of said hollow piston portion, whereby there is no frictional wear of the circumferential surface of said piston portion and the facing inner surface of said cylinder, and wherein said cylinder is further provided with an axially extending guide portion for guiding said piston Rod portion, said guide portion having an axial end portion facing said piston portion, and wherein said piston rod portion is further provided with a recess constituting a suction release passage having one end opening into the ambient atmosphere and having another end which terminates just past said axial end portion of said guide portion when said piston unit is in said retracted position.
 2. In a sheet-handling machine, a suction device for engaging and then holding sheets, comprising, in combination, a pneumatic cylinder; a hollow piston unit in said cylinder movable between a retracted and a non-retracted position and comprised of axially adjacent first and second portions, at least a part of the circumferential periphery of each of said portions being spaced from and out of contact with the inner periphery of said cylinder to define therewith respective first and second spaces, said piston unit including a sheet-engaging portion having a suction opening which communicates with the ambient atmosphere and constitutes the end of the hollow interior of said piston unit, said hollow interior communicating with said second space via said first space, said cylinder being provided with a suction port opening into said second space to form a path for the flow of ambient air into said suction opening and out through said port, the flow cross section of said first space being smaller than that of said second space so that said first space constitutes a flow restriction in said path; and drive means for causing said piston unit to move from retracted to non-retracted position and then back to retracted position, in order to cause said sheet-engaging portion to move into engagement with a sheet located in the path of movement of the latter and then hold said sheet by suction during the movement of said piston unit back to said retracted position, said drive means comprising suctiongenerating means connected to said port and operative when activated for establishing subambient pressure in said second space, whereby said piston unit moves from said retracted to said non-retracted position upon the establishment of such subambient pressure as a result of the initial difference in pressures upstream and downstream of said first space, and whereby said piston unit returns to said retracted position due to the pressure equalization resulting when the sheet engaged and held by said sheet-engaging portion blocks further entrance of ambient air into said suction opening of said sheet-engaging portion, the hollow interior of said piston unit and the part of the hollow interior of said cylinder external to said piston unit together forming when said suction opening is blocked by a sheet and said piston unit is in the retracted position a closed chamber sealed off from the ambient atmosphere to such an extent that the subambient pressure established in such a chamber by said suction-generating means maintains said piston unit in the retracted position indefinitely until said suction-generating means is deactivated.
 3. In a sheet-handling machine as defined in claim 2, wherein said first space is annular and wherein said second space is annular.
 4. In a sheet-handling machine as defined in claim 2, wherein said first and second portions of said piston unit are cylindrical and coaxial, with the diameter of said first portion being larger than the diameter of said second portion, and wherein said first and second spaces are both annular.
 5. In a sheet-handling machine as defined in claim 2, wherein at least one of said first and second portions of said piston unit is spaced from and out of contact with the inner periphery of said cylinder all around the circumferential periphery of said at least one of said portions.
 6. In a sheet-handling machine as defined in claim 2, wherein at least said first portion of said piston unit is spaced from and out of contact with the inner periphery of said cylinder all around the circumferential periphery of said first portion.
 7. In a sheet-handling machine as defined in claim 2, wherein at least said second portion of said piston unit is spaced from and out of contact with the inner periphery of said cylinder all around the circumferential periphery of said second portion.
 8. In a sheet-handling machine as defined in claim 2, wherein said first space constitutes the only flow restriction in said path.
 9. In a sheet-handling machine as defined in claim 2, wherein said cylinder is further provided with a suction release opening having a flow cross-sectional area so related to that of said second space and to the magnitude of said subambient pressure in said chamber that when a sheet engaged and held by said sheet-engaging portion blocks off said suction opening the loss of suction inside said hollow piston unit is insufficient to result in termination of the holding action, but so related to the volume of said second space that upon deactivation of said suction-generating means ambient air entering said suction release passage can quickly destroy the subambient pressure in said second space.
 10. In a sheet-handling machine as defined in claim 2, wherein said first portion of said piston unit is cylindrical and constitutes a hollow piston portion and wherein said second portion of said piston unit is cylindrical and constitutes a hollow piston rod portion, and wherein said hollow piston portion is spaced from said out of contact with the inner periphery of said cylinder all around the circumferential periphery of said hollow piston portion, whereby there is no frictional wear of the circumferential surface of said piston portion and the facing inner surface of said cylinder, and wherein said cylinder is further provided with an axially extending guide portion for guiding said piston rod portion, said guide portion having an axial end portion facing said piston portion, and wherein said piston rod portion is further provided with a recess constituting a suction release passage having one end opening into the ambient atmosphere and having another end which terminates just past said axial end portions of said guide portion when said piston unit is in said retracted position. 